1. Field of the Invention
The present invention relates to a magnetic head having horizontal pole pieces. It is used in magnetic recording.
2. Discussion of the Background
A known magnetic head is shown in section in FIG. 1 and in plan view in FIG. 2. On a substrate 8, said head has a first subassembly 10 constituted by a lower magnetic layer 12, two magnetic pillars 14.sub.1, 14.sub.2 resting on said magnetic layer 12 and a conductor winding 18 surrounding the pillars 14.sub.1, 14.sub.2. On said subassembly 10 there is a central, insulating pin or ridge 19 having two inclined sides and a flat apex. On the subassembly 10 and overlapping the central, insulating ridge 19 there is a second subassembly 20 formed from two portions, namely a first portion constituted by two magnetic flux concentrators 22.sub.1, 22.sub.2 with an outer, wide end bearing on the pillars 14.sub.1, 14.sub.2 and an inner, narrower end, as well as a second portion constituted by two pole pieces 24.sub.1, 24.sub.2 separated by a head gap 26 and in contact with the concentrators 22.sub.1, 22.sub.2. These pole pieces have an elongated S shape with an oblique median portion covering the inclined sides of the pin 19.
FIG. 3 shows in detailed manner the right-hand half of the second subassembly 20. This drawing makes it possible to define a few dimensions and a few particular zones. The pole pieces have a thickness Ee at the head gap 26 and Ep remote from the head gap. The ridge or pin 19 has a half-length Lp. The concentrators 22.sub.2 have a thickness Ec. The movement plane of the head is at a distance H from the upper surface of the concentrators. The angle formed with the horizontal by the pole piece in its inclined portion is designated .theta.. Moreover, the point G designates the centre of the head gap, the point A marks the limit of the straight zone of a pole and point P corresponds to the zone where the pole piece leaves the movement plane and becomes inclined.
Although these heads are satisfactory in certain respects, they still suffer from disadvantages when the width of the recording track becomes small and the operating frequency high. These different disadvantages can be analyzed in the following way.
The construction of the pole pieces imposes a monolithic structure for the same. With a considerable thickness of the pole pieces, eddy currents appear and create heating, as well as a reduction of efficiency. By reducing the thicknesses Ep and Ec, this effect is reduced, but to the detriment of the overall efficiency, as well as the maximum field obtained in the head gap under saturation conditions. Thus, there is a maximum and minimum limitation for the thickness Ep.
As shown in FIG. 4, the reading signal, apart from the main peak Pp, has two small secondary peaks Ps1, Ps2 at a distance of approximately L corresponding to point P in FIG. 3, where the poles are no longer in the movement plane. The height of these parasitic peaks is also dependent on the angle .theta. obtained during the production of the ridge or pin.
In order to obtain a maximum field in the head gap on writing, it is necessary to move the point A (saturation appearance point) towards the head gap. It is then necessary to reduce the half-length Lp of the ridge without modifying the optimum angle .theta., which must remain small in order to ensure the quality of the magnetic deposit. This reduction is difficult for lengths below 5 .mu.m. Moreover, the production of the head gap on a ridge of limited length also becomes very difficult.
Thus, the track width reduction and the obtaining of a larger field in the head gap require the reduction of the size of the relief and the retention of a small angle .theta.. This leads to technological problems in producing the ridge and the head gap for small track widths.
In order to avoid parasitic rereading of adjacent tracks, the poles must have a width equal to that of the track to be read and the introduction of a concentrator with a wide section towards the pillars and a narrow section towards the head gap makes it possible to retain a good efficiency. However, a relatively large height H is necessary to avoid any rereading in the presence of the wide section of the concentrator. Moreover, at high frequency, the considerable thickness Ep+Ec of the pole pieces and the concentrator is prejudicial with respect to eddy currents and it is necessary to reduce each thickness and magnetically decouple the layers.